For conventional two-stroke combustion engines it is well known how to control the air/fuel mixture, but a high level of unburned hydrocarbon emissions is produced because in this engine the scavenging process is performed exclusively by a mixture of air and fuel. Some of the air/fuel mixture mixes with the exhaust gases and thereby some unburned fuel is lost out with the exhaust gases.
Two-stroke combustion engines using the additional air technique during the scavenging are previously known in the art. These engines reduce fuel consumption and exhaust emissions. But for these prior art engines it is a problem to control the air/fuel mix during normal operation and during start. One common start procedure for a conventional two-stroke engine is with a closed choke valve and a fully open throttle valve. In combination with a “conventional” additional air technique this will provide too much air for optimal starting conditions. Since these kind of engines are used in many different environments and subjected to troublesome conditions it is desired to achieve a reliable design with a robust operation. One issue with engines using the additional air technique is to also provide good start properties. The operational conditions for this kind of engines can be of varying temperature, humidity, atmospheric pressure etc. The manufacturer Walbro has launched a carburettor comprising a double barrel valve with one part for the mixture and one part for the additional air. This however is a rather complicated design and much more expensive than a conventional carburettor. U.S. Pat. No. 6,328,288 shows a system comprising two butterfly valves. An elongated slot is arranged in the throttle valve lever for fastening the link to the air valve lever to give a delayed opening of the air valve. This arrangement of a link is not very satisfactory as there is an increased risk that the link will fall apart. Further there is a risk of ice build-up in the slot in the winter. This will make starting very difficult.
Hence, there is a need for an improved system for a two-stroke engine using the additional air technique to achieve good properties for start and normal operation. Further it is an advantage to combine proper operation of such an engine with lean and environmentally friendly operation. Finally it would be advantageous to provide a robust, cost effective and high-quality system for this kind of engine.